Mercruiser 7.4L Overheat

[b0mbtrack]

Mercruiser, 7.4L MPI. I will get the engine number tomorrow but it is the kind with the thermostat in the heat exchanger, I think they only did that for a couple of years if that helps. Freshwater cooled, damper pulley mounted raw water pump.

I can start the engine up and let it run all day at the dock and idle up the creek with no problem. When I get out on the water and do full throttle it will get hot after running for about 5 minutes or so. However, it really starts to overheat after I pull it down to idle from this full throttle.
Things I have done so far: Changed the raw water pump (housing as well), changed the thermostat, replaced manifolds and risers (for another issue), drilled open the holes in the intake screen, found and fixed a few antifreeze leaks.
The next thing I am going to do is to take off the heat exchanger and the oil cooler and boil them. I just thought I would post this to see what anyone else thinks.

 

[b0mbtrack]

Re: Mercruiser 7.4L Overheat

Ok I don't have the exact number but it looks to be in OL01003 to OM024330 family.

 

[b0mbtrack]

Re: Mercruiser 7.4L Overheat

I just pressure tested the heat exchanger and cooling system at 20psi for 2 minutes and it stayed at 20psi the whole time so at least I know it is not a leak or blown head gasket or something.

 

[Fireman431]

Re: Mercruiser 7.4L Overheat

As far as I can remember, don't the 7.4L MPI's also run an engine mounted water pump like a standard auto engine would? If so, that would be my guess. If the impeller is shot, it would be moving just enough water at an idle to keep it cooled off. Once it got hot from running and backed down to an idle, there wouldn't be enough water circulating to allow it to cool back to temp and it's getting hotter.

Just a thought...

 

[b0mbtrack]

Re: Mercruiser 7.4L Overheat

This one used to have the normal style pump on it that was turned by a belt but was switched over to this style many years ago. I'm not sure why it was switched over though, but it was running fine on the new style damper mounted pump all these years. I changed the pump impeller and the housing and everything to do with the pump to rule that out though.

I did just remove the heat exchanger, power steering cooler, and oil cooler but they all look fine. I flushed them out and the water flows through nicely. I have most of the hoses off and they all look ok. I'll probably replace one with a spring in it, the spring looks old. I took the drain screws out of the block and they were real clogged so I unclogged them and the anti-freeze poured out. So I guess I will boil these heat exchangers, put it all back together and run the engine on some sort of cooling system cleaner because everything looks ok.

 

[b0mbtrack]

Re: Mercruiser 7.4L Overheat

I think I might be good but thought I would check to see what you guys think. I just got done boiling out heat exchanger, I checked all the lines on both the fresh water and salt water side. I cleaned out the power steering cooler and the oil cooler. I drained the entire system and ran it on regular fresh water with a cleaner in it then drained that and put in the anti-freeze.
My gauge has no temp numbers on it, just lines so I used my temp gun on the engine to see how it was doing. The engine did get better but I think it might still be just a little hot.
Top of risers: about 90*
thermostat housing: about 180* (located on heat exchanger)
where thermostat would normally be: about 175* (front of engine on intake manifold)

The thermostat is a 160* like it calls for and like i said the temp is better than it was but not as good as I was hoping. I can idle all day fine in gear and I can do full throttle for a while but when I bring it down to troll around is when it starts to rise a bit more to these temps and stays there. Let me know what you think, thanks.

 

[Bt Doctur]

Re: Mercruiser 7.4L Overheat

head gasket allowing hot exaust gas into the cooling system.

 

[b0mbtrack]

Re: Mercruiser 7.4L Overheat

I did pressurize the cooling system and didn't find any leaks. It held good and didn't drop. I forget but I think it was either 15 or 20psi that I did it at. According to the book you do that with the engine off and cold which is what I did but just double checking.

 

[Bt Doctur]

Re: Mercruiser 7.4L Overheat

you can test a block for cracks like that but the pressure at firing goes up over 2000psi

 

[b0mbtrack]

Re: Mercruiser 7.4L Overheat

oh ok, I'll have to do the test with the clear hose from the heat exchanger to the recovery bottle then to check for bursts of air at a higher rpm, what do you think?

 

[Bt Doctur]

Re: Mercruiser 7.4L Overheat

clear hose woould show any bubbles coming into the system

 

[ENSIGN]

Re: Mercruiser 7.4L Overheat

How much water is flowing from your seawater pump? On average it's normaly about 8 quarts in 15 seconds at idle speed.

 

[HT32BSX115]

Re: Mercruiser 7.4L Overheat

thermostat housing: about 180* (located on heat exchanger)
where thermostat would normally be: about 175* (front of engine on intake manifold)

The thermostat is a 160* like it calls for and like i said the temp is better than it was but not as good as I was hoping. I can idle all day fine in gear and I can do full throttle for a while but when I bring it down to troll around is when it starts to rise a bit more to these temps and stays there. Let me know what you think, thanks.

Howdy,


I have a closed cooled 454 in my Lib. It's a San Juan Engr after market system that uses their own T-stat housing and also uses a 160* stat.

I routinely see 170-180 on the gage.........and the sender is located in the housing.

I also have an ( Harbor Freight) infrared "gun" but haven't checked it.

As far as I can remember, don't the 7.4L MPI's also run an engine mounted water pump like a standard auto engine would? If so, that would be my guess. If the impeller is shot,

Since the engine mounted circulating pump only pumps 50/50 AF mix, the impeller would outlast the boat, engine, and you!! The failure mode would be to just start leaking at the seal......It would always continue to pump coolant.....

As long a you use 50/50 ATF, you could even use a regular 454 truck water pump and it would work just as good and as long as a "marine" pump.....

However, it really starts to overheat after I pull it down to idle from this full throttle.

What does this mean? what's the temp? without a real gage, I don't think you know if it's really overheating or not...



Put a direct reading gage on it and run it that way before you start replacing stuff.....


Cheers,


Rick


If your temps not going above 180 on the gage, I don't think you have a problem.

 

[b0mbtrack]

Re: Mercruiser 7.4L Overheat

hey HT32BSX115, I've been using a heat gun to see the temp. But, its fixed and you are all going to laugh at me and I deserve it. I went to run the engine on the clear hose and this is the first time I actually ran the engine full speed while looking at it as I had to remove the engine box to see the hose. Well every time I would slow down anti-freeze would pour out of the top of the cap. So I went back and fixed up the cap and the surface of the heat exchanger (put an extra gasket in there) and went back out. It no longer poured out of the top of the cap this time when I slowed down, there were no air bubbles in the hose, and the engine was at the correct temp. Thanks all for the help, now I just got to find a better gasket to put under the cap but the one I have now is good for now.

 

[HT32BSX115]

Re: Mercruiser 7.4L Overheat

But, its fixed and you are all going to laugh at me and I deserve it.

Well,


I never laugh at anyone since I have probably done as many "dumb" things as the next guy!


We all sometimes learn the hard way!!